The invention relates to a pump assembly for a slip-controlled hydraulic brake system for a vehicle.
One such pump assembly is known from German Patent Disclosure DE 42 39 361 A1. As its pump motor, the known pump assembly has an electric motor, on whose motor shaft an eccentric element is mounted in a manner fixed against relative rotation. A pump piston of a piston pump of the pump assembly rests with an end face on a circumference of the eccentric element. The pump piston is received axially displaceably in a cylinder bore of a pump housing. The pump piston can be guided axially displaceably in the pump housing directly, or in a bush inserted into the pump housing. When the eccentric element is driven to rotate by the pump motor, the pump piston resting on the circumference of the eccentric element is driven to execute an axially reciprocating stroke motion in the cylinder bore, which in a manner known per se for piston pumps brings about pumping of brake fluid. As the eccentric element, a crank (crankshaft) can for instance also be used, which drives the pump piston via a connecting rod.
The eccentric element is disposed in an eccentric element chamber of the pump housing into which the cylinder bore discharges. The pump motor is mounted on or in the pump housing. The piston pump has a seal toward the eccentric element, near the eccentric element chamber, and this seal is intended to prevent brake fluid, to be pumped by the piston pump, from overflowing into the eccentric element chamber. This seal may have a sealing element, such as a sealing ring, and sealing can also take place directly between the cylinder bore and a circumferential surface of the pump piston without any separate sealing element. This too should be understood by the term xe2x80x9csealxe2x80x9d in the sense of the present invention. Despite the seal, brake fluid can flow out of the piston pump as a leakage flow between the cylinder bore and a circumferential face of the pump piston into the eccentric element chamber. On the one hand, because of its stroke motion, the pump piston can pump brake fluid adhering to its circumferential surface past the seal into the eccentric element chamber, in the form of so-called drag flow, and on the other, the sealing action of the seal can lessen over time from wear. To prevent brake fluid from accumulating in the eccentric element chamber, the pump housing of the known pump assembly is provided with a bore as a drain, which leads downward from the eccentric element chamber into the open air. To prevent water from entering, a check valve is disposed in the drain.
The known pump assembly has the disadvantage that the pump assembly additionally requires the check valve, and that the check valve must be inserted into the drain during the assembly of the pump assembly, which means additional assembly effort and expense. Invading dirt can moreover impair the closing function of the check valve.
From German Patent Disclosure DE 197 12 686 A1, a pump assembly of the type described above is known, in which a porous sintered metal body, which is therefore permeable to the brake fluid, is inserted into an opening that forms the drain. This pump assembly has the disadvantage that oil can get into the drain from an eccentric element bearing or motor bearing. This oil lessens the permeability of the sintered metal body and stops up the sintered metal body entirely with time.
The two known pump assemblys both have the disadvantage that leaking brake fluid from the piston pump first gets into the eccentric element chamber before flowing into the drain. From the eccentric element chamber, the brake fluid can reach the eccentric bearing or penetrate into the electric motor. The brake fluid destroys a lubricating film on the bearings, thus leading to their destruction. In the electric motor, the brake fluid can cause a short circuit and thus the destruction of the electric motor.
In the pump assembly according to the invention the drain discharges into the cylinder bore, between the seal toward the eccentric element and the eccentric element chamber. Leaking fluid from the piston pump that has overcome the seal toward the eccentric element flows through the drain out of the cylinder bore before reaching the eccentric element chamber. This prevents leaking fluid from the piston pump from reaching the interior of the eccentric element chamber. As a result, mixing of leaking fluid (brake fluid) with lubricant or bearing oil of an eccentric element bearing or motor bearing, which is highly undesirable, does not occur. Since no leaking fluid from the piston pump reaches as far as the eccentric element chamber, leakage fluid is prevented from reaching an eccentric element bearing or motor bearing or the pump motor. Damage to an eccentric element bearing or motor bearing or the pump motor from brake fluid is thus precluded. Oil from an eccentric element bearing or motor bearing is prevented from reaching the drain and being able to stop up a sintered body that might be present. The drainage of leakage fluid is thus assured permanently.
The pump assembly of the invention is intended in particular as a pump in a brake system of a vehicle and is used to control the pressure in wheel brake cylinders. Depending on the type of brake system, the abbreviations ABS (for anti-lock brake system), TCS (traction control system), ESP (electronic stability program) and EHB (electrohydraulic brake system) are used for such brake systems. In the brake system, the pump serves for instance to return brake fluid from a wheel brake cylinder or a plurality of wheel brake cylinders to a master cylinder (ABS) and/or to pump brake fluid out of a supply container into a wheel brake cylinder or a plurality of wheel brake cylinders (TCS or ESP or EHB). The pump is needed in a brake system with wheel slip control (ABS or TCS) and/or a brake system serving as a steering aid (ESP) and/or an electrohydraulic brake system (EHB). With wheel slip control (ABS or TCS), locking of the wheels of the vehicle during a braking event involving strong pressure on the brake pedal (ABS) and/or spinning of the driven wheels of the vehicle in the event of strong pressure on the gas pedal (TCS) can for instance be prevented. In a brake system serving as a steering aid (ESP), a brake pressure is built up in one or more wheel brake cylinders independently of an actuation of the brake pedal or gas pedal, for instance to prevent the vehicle from breaking out of the track desired by the driver. The pump can also be used in an electrohydraulic brake system (EHB), in which the pump pumps the brake fluid into the wheel brake cylinder or wheel brake cylinders if an electric brake pedal sensor detects an actuation of the brake pedal, or in which the pump is used to fill a reservoir of the brake system.
The disposition of a liquid-permeable ring in a groove surrounding the pump piston in accordance with invention has the advantage that the liquid-permeable ring is disposed on an inner or piston end of the drain and is thus protected against water.